Outstanding Exposure - Financial Guaranty Portfolio by Asset Class (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	$ 269,386	$ 307,474
Net Par Outstanding	264,952	296,318
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	256,301	281,392
Net Par Outstanding	252,314	271,179
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	13,085	26,082
Net Par Outstanding	12,638	25,139
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	220,616
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	211,441	250,905
Net Par Outstanding	209,392	244,798
United States [Member] | Public Finance [Member] | General obligation [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	91,531	110,167
Net Par Outstanding	90,705	107,717
United States [Member] | Public Finance [Member] | Tax backed [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	44,783	51,325
Net Par Outstanding	44,350	49,931
United States [Member] | Public Finance [Member] | Municipal utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	32,584	38,442
Net Par Outstanding	32,357	37,603
United States [Member] | Public Finance [Member] | Transportation [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	17,193	19,915
Net Par Outstanding	17,030	19,403
United States [Member] | Public Finance [Member] | Healthcare [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	9,087	11,940
Net Par Outstanding	8,763	11,238
United States [Member] | Public Finance [Member] | Higher education [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	8,210	10,114
Net Par Outstanding	8,195	10,085
United States [Member] | Public Finance [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	4,259	3,902
Net Par Outstanding	4,216	3,769
United States [Member] | Public Finance [Member] | Housing [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,336	1,593
Net Par Outstanding	1,319	1,559
United States [Member] | Public Finance [Member] | Investor-owned utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	523	697
Net Par Outstanding	523	697
United States [Member] | Public Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,935	2,810
Net Par Outstanding	1,934	2,796
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	11,652	22,731
Net Par Outstanding	11,224	22,057
United States [Member] | Structured Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	461	689
Net Par Outstanding	456	640
United States [Member] | Structured Finance [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,347	10,273
Net Par Outstanding	1,347	10,050
United States [Member] | Structured Finance [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	4,864	5,933
Net Par Outstanding	4,818	5,637
United States [Member] | Structured Finance [Member] | Consumer receivables [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,591	1,707
Net Par Outstanding	1,590	1,652
United States [Member] | Structured Finance [Member] | Insurance Securitizations [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,825	2,355
Net Par Outstanding	1,449	2,308
United States [Member] | Structured Finance [Member] | Financial product [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,418	1,540
Net Par Outstanding	1,418	1,540
United States [Member] | Structured Finance [Member] | Commercial receivables [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	146	234
Net Par Outstanding	146	230
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	44,336
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	44,860	30,487
Net Par Outstanding	42,922	26,381
Non United States [Member] | Public Finance [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	18,916	11,818
Net Par Outstanding	18,234	10,731
Non United States [Member] | Public Finance [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	17,691	11,395
Net Par Outstanding	16,689	9,263
Non United States [Member] | Public Finance [Member] | Pooled infrastructure [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,561	1,621
Net Par Outstanding	1,561	1,513
Non United States [Member] | Public Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	6,692	5,653
Net Par Outstanding	6,438	4,874
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,433	3,351
Net Par Outstanding	1,414	3,082
Non United States [Member] | Structured Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	325	601
Net Par Outstanding	324	587
Non United States [Member] | Structured Finance [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	157	1,716
Net Par Outstanding	157	1,535
Non United States [Member] | Structured Finance [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	655	661
Net Par Outstanding	637	604
Non United States [Member] | Structured Finance [Member] | Commercial receivables [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	296	373
Net Par Outstanding	$ 296	$ 356